I have written my thesis for A4 paper using the memoir class. I would like to print one version on A5 paper. Is there a way to reduce margins automatically in order to keep the same layout for the text?

I mean, I have for instance 70 characters per line, I would like to keep this number of characters per line by decreasing the size of the margin.

Why don't you use pdfpages and \includepdf?
– user11232Dec 10 '14 at 11:16

Unless you used unusual large margins the textwidth in the A4 paper is larger than a A5 paper, so it can't work without changing the font size.
– Ulrike FischerDec 10 '14 at 11:37

Sorry I was not clear enough. I would like to keep the same font size and picture size (or something close at least). I used the standard margins of memoir class that are quite large for A4. So I would like to decrease as much margins as possible. It does not matter if I have more pages at the end. I just want to keep something easy to read. note that I will print an official version in A4, the A5 version is for me.
– nbiDec 10 '14 at 12:19